Liquid crystal display devices utilize the electrooptical effect of liquid crystals, and the liquid crystal phases used in them include the nematic phase, the cholesteric phase, and the smectic phase. The display system which is now most widely used is a twisted nematic (hereinafter referred to as TN) type system of utilizing the nematic phase, or a super twisted nematic (hereinafter referred to as STN) type system in which the twisted angle is enlarged.
Because liquid crystal display devices advantageously: i) are small and thin; ii) have a low driving voltage and small power consumption; and iii) do not cause eyestrain even after use for an extended period of time as a result of the image-receiving element, they have heretofore found application in watches, electronic calculators, audio instruments, various metering instruments and car dashboard displays. Recently, in particular, they have been utilized also in displays of personal computers and word processors as well as in color televisions or the like having an extremely large number of pixels, and are therefore especially useful as display devices substitutable for a cathode ray tube. Thus, liquid crystal display devices now find application in various fields, and should find expanded usage in the future.
The following characteristics of liquid crystal materials are considered to be basic and indispensable:
1. Liquid crystal materials are colorless; stable under exposure to heat and light; and stable both electrically and chemically. PA1 2. They have a broad practical temperature range. PA1 3. They have a rapid electrooptical response speed. PA1 4. They require a low driving voltage. PA1 5. The increase of their voltage-light transmittance characteristic is rapid, and the temperature dependence of their threshold voltage (V.sub.th) is small. PA1 6. They have a broad viewing angle range.
Many conventional liquid crystals are capable of providing characteristic 1, but liquid crystal compounds capable of providing characteristics 2 to 6 in a single component are unknown.
In order to obtain these characteristics, liquid crystal compositions each comprising nematic liquid crystal compounds of plural types or containing both non-liquid crystal compounds and liquid crystal compounds are used. In order to provide characteristic 2, liquid crystal compounds having a low crystal nematic phase transition point (hereinafter referred to as C-N point) and having a high nematic isotropic phase transition point (hereinafter referred to as N-I point) and, as a result, having a broad nematic mesomorphic range, are needed.
With respect to characteristic 3 (response speed, hereinafter referred to as .tau.), the viscosity coefficient (hereinafter referred to as .eta.) and the cell gap (hereinafter referred to as d) have the following relative expression: EQU .tau..alpha..eta.d
Therefore, in order to satisfy characteristic 3, d must be small so as to increase the response speed. In cells for practical use, since the value of .DELTA.n.cndot.d (where .DELTA.n represents the refractive index anisotropy) is determined to be a constant value in order to prevent generation of interference fringes on the surface of the cell, which would worsen the outward appearance of the cell, the value of d may be reduced when a material having a large .DELTA.n value is used and, as a result, the response speed may be increased. In order to provide both characteristics 2 and 3, cyclohexyltolan derivatives and phenyltolan derivatives are known as liquid crystal materials having a broad nematic liquid crystal range and a large .DELTA.n value, as described in JP-A 60-155142 and 60-152427, U.S. Pat. No. 4,713,468 and JP-A 63-152334. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
TABLE 1 __________________________________________________________________________ Structural Formula Transition Point Reference __________________________________________________________________________ a-1 ##STR2## CN point 135.degree. C. NI point 250.degree. C. JP-A 60-155142 a-2 ##STR3## CN point 153.degree. C. NI point 265.degree. C. JP-A 60-155142 b-1 ##STR4## CN point 96.degree. C. NI point 213.degree. C. JP-A 60-152427 b-2 ##STR5## CN point 87.degree. C. NI point 201.degree. C. US Pat. No. 4,713,468 c-1 ##STR6## CN point 79.degree. C. NI point 199.degree. C. JP-A 63-152334 c-2 ##STR7## CN point 50.degree. C. NI point 188.degree. C. JP-A 63-152334 d-1 ##STR8## CN point 164.degree. C. NI point 230.degree. C. JP-A 60-152427 d-2 ##STR9## CN point 160.degree. C. NI point 211.degree. C. JP-A 60-152427 __________________________________________________________________________
Of these compounds, however, cyclohexyltolan derivatives of (a-1) to (c-2) each have a cyclohexane ring in the skeleton, and the ring interferes with the effect of enlarging the value .DELTA.n. Compounds (a-1) and (a-2) each have a high C-N point and therefore are considered to have a compatibility problem with other liquid crystal compounds. Compounds (d-1) and (d-2), though having a large .DELTA.n value, also have an extremely high C-N point and therefore are also considered to have a compatibility problem.